Transaction Inputs and Fusion Transactions (or “What is the max amount of TRTL I can send?”)

A large TurtleCoin transaction

A common question we get from users is wondering what the maximum amount of TRTL they can send is, often because they went to send a (relatively) small amount, and got an error ‘Transaction is too big’.

The question seems pretty simple, but the answer is not quite so simple!

To begin with, lets talk about what a transaction is made of.

Transaction Inputs / Outputs

When you send a transaction, lets say, 1000 TRTL, you might think it’s as simple as taking 1000 TRTL from your balance, encrypting it with your recipient’s public keys, and sending it out to the miners, to be included in a block.

However, your 1000 TRTL is actually made up of lots of smaller ‘Inputs’. These inputs come from previous transactions that you have recieved.

To demonstrate what I mean by this, lets start by looking at a coinbase transaction. A coinbase transaction is a special kind of transaction, where there is no sender. These are the rewards a miner gets for finding a block.

A coinbase transaction
The transaction the miner of block 800,000 received

Here’s the transaction that the miner of block 800,000 found. As you can see, they received 29,100.54 TRTL, but, if we look at the outputs section, they did not receive it all in one lump.

The transaction has been split up into several standard sized ‘Outputs’. The reason for doing this links into the privacy elements of TurtleCoin, and other CryptoNote coins. When we send a transaction, we hide one of our inputs alongside many other inputs. The network can verify that we own one of the inputs, but not which one. This allows the sender of a transaction to be hidden.

For this to work, we need there to be other inputs to select from. If we simply sent the exact amount in every transaction, then if you wanted to send an amount that had never been sent before, say, 1337 TRTL, you would not be able to obscure that you were the sender.

To ensure we always have enough inputs to match with, we use some set values, and build our transactions out of these smaller sized building blocks.

If you’re interested, you can view the possible sizes for transaction inputs here.

Hold on, what does it matter if my transaction has lots of inputs?

Well, when we send a transaction, we have to include each of our transactions inputs, along with the decoy inputs from other users, for the privacy features to function. These need to be available so we can verify that a user isn’t spending funds they don’t own, or trying to spend an input that has already been used.

To use a real world example, if you try and pay for a car in pennies, you’d need a pretty large jar to hold them all in. Similarly, when sending a transaction, each input takes up a bit of space in the blockchain. The largest a transaction can be is approximately 140 KB, and if your transaction will be larger than this, your wallet will reject it. This prevents your transaction being too large for a block, and never getting mined.

If your transaction is too large, stop thinking in amount, and start thinking in bytes.

Change

An unoptimized TurtleCoin wallet

Hopefully now you understand that a transaction is comprised of multiple inputs to make up the full amount of your transaction. One question on your mind may be – What if I have an input of 2000 TRTL, but I only want to send 1500 TRTL?

Well, we can do the exact same thing as when you were a kid and wanted to buy a candy bar, but you only had 5 dollars – you give the cashier your 5 dollars, and she gives you back 4 dollars in change.

Here’s an example of a real transaction, to show what I mean.

In this image, I sent 1500 TRTL to someone. You can see I used one input, of 20000 TRTL to do so. 500 TRTL + 1000 TRTL went to the receiver, and the other 18500 TRTL got sent back to my wallet.

You may have noticed when you send a transaction, some of your balance gets ‘Locked’. This is because you have had to send a bit more of your funds than needed, as you cannot make the exact amount, and you are waiting for the change to confirm on the blockchain, and return to you.

Optimization / Fusion Transactions

A TurtleCoin user performing fusion transactions

Now lets talk a bit about fusion transactions. If you haven’t heard of these before, that’s OK, they’re quite simple.

Recall how when we send a transaction, as long as the total value of the inputs/outputs on both sides match up, the amount of inputs/outputs we have doesn’t matter?

A fusion transaction is a special kind of transaction, where you are sending a transaction to yourself. We take all our small inputs on one side, and combine them into a few large outputs.

It’s a bit like those machines where you pour in your loose change, and get out a few dollar bills.

How can I send more TRTL?

So, on to the final section of this article – putting these steps into practice, so you can send more TRTL at one time.

Sending fusion transactions

Our first step we can take is to perform fusion transactions. The manner to do this depends upon what wallet you are using.

If you are using zedwallet, you can type optimize to perform multiple fusion transactions. This will also be done automatically for you, if a transaction fails due to being too large.

Similarly in Nest, if a transaction fails, it will offer you the chance to send a fusion transaction. These are done one at a time, so may take quite a while if you have a very unoptimized wallet!

Splitting up our transactions

If you’ve performed fusion transactions, and still aren’t able to send very large amounts, then unfortunately, the best option you can do is to split your transaction up into multiple smaller transactions.

Again, zedwallet will do this automatically for you (If you agree) upon failing to send a transaction due to it being too large. In Nest however, you will have to do this manually, with trial and error.

Avoiding the issue

Now, you might be wondering why you have so many small inputs. The simple answer is you have recieved a lot of transactions, or more importantly, a lot of small transactions. If you receive 1000 TRTL at a time, then the largest a single input could be, is 1000 TRTL. If you then want to send 100,000 TRTL, you will need to include 100 inputs, which ends up as a pretty large transaction.

If your mining pool has the option, you can increase your payment threshold. This will result in fewer inputs in your wallet, each with a larger amount. 1 payout of 5000 TRTL instead of 5 payouts of 1000 TRTL may make your transactions around 5 times smaller!

Solo miners can often send more TRTL at once

You may also like to consider using a smaller pool, which due to taking a longer time to find blocks, pays you out in larger chunks. Special mention to http://cryptonote.social/ which is a very cool solo mining pool – You get a full block payout everytime.

This is one of the reasons you encounter this issue in TurtleCoin, but don’t in other currencies, such as Monero, or Bitcoin – The payout thresholds for these currencies on mining pools are often very high, so you have very few transactions received. It’s worth noting that most cryptocurrencies use this method of including past payments in your payments, and returning extra change as a transaction to yourself.

If you’d like to read up more on the topic, in Bitcoin, these are called ‘Unspent Transaction Outputs’, or UTXOs.

Thanks

  • Thanks to turtle.land for the transaction screenshots – it’s a very nice looking block explorer, with great uptime!
  • Thanks to Der Wixer for his optimize maymay

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